1. Field of the Invention
The present invention relates generally to an apparatus and method for measuring a propagation delay in a CDMA (Code Division Multiple Access) mobile communication system, and in particular, to an apparatus and method for measuring a propagation delay in an NB-TDD (Narrow Band Time Division Duplexing) CDMA mobile communication system.
2. Description of the Related Art
Presently, the mobile communication system has evolved from an early voice-based communication system to a high-speed, high-quality radio data packet communication system for providing a data service and a multimedia service. In addition, a 3rd generation mobile communication system, divided into an asynchronous 3GPP (3rd Generation Partnership Project) system and a synchronous 3GPP2 (3rd Generation Partnership Project 2) system, is on the standardization for a high-speed, high quality radio data packet service. For example, standardization on HSDPA (High Speed Downlink Packet Access) is performed by the 3GPP, while standardization on 1×EV-DV (1× Evolution-Data and Voice) is performed by the 3GPP2. Such standardizations are implemented to find out a solution for a high-speed, high-quality radio data packet transmission service of 2 Mbps or over in the 3rd generation mobile communication system. Further, a 4th generation mobile communication system has been proposed, which will provide a high-speed, high-quality multimedia service superior to that of the 3rd generation mobile communication system.
A principal factor that impedes a high-speed, high-quality radio data service lies in the radio channel environment. The radio channel environment frequently changes due to a variation in signal power caused by white nose and fading, shadowing, Doppler effect caused by a movement of and a frequent change in speed of a UE (User Equipment), and interference caused by other users and a multipath signal. Therefore, in order to provide the high-speed radio data packet service, there is a need for an improved technology capable of increasing adaptability to variations in the channel environment in addition to the general technology provided for the existing 2nd or 3rd generation mobile communication system. A high-speed power control method used in the existing system also increases adaptability to variations in the channel environment. However, both the 3GPP and the 3GPP2, carrying out standardization on the high-speed data packet transmission, make reference to AMCS (Adaptive Modulation/Coding Scheme) and HARQ (Hybrid Automatic Repeat Request).
The AMCS is a technique for adaptively changing a modulation technique and a coding rate of a channel encoder according to a variation in the downlink channel environment. Commonly, to detect the downlink channel environment, a UE measures a signal-to-noise ratio (SNR) and transmits the SNR information to a Node B over an uplink. The Node B predicts the downlink channel environment based on the received SNR information, and designates a proper modulation technique and coding rate according to the predicted value. The modulation techniques available for the AMCS include QPSK (Binary Phase Shift Keying), 8PSK (8-ary Phase Shift Keying), 16QAM (16-ary Quadrature Amplitude Modulation), and 64QAM (64-ary Quadrature Amplitude Modulation), and the coding rates available for the AMCS include ½ and ¾. Therefore, an AMCS system applies the high-order modulations (16QAM and 64QAM) and the high coding rate ¾ to the UE located in the vicinity of the Node B, having a good channel environment, and applies the low-order modulations (QPSK and 8PSK) and the low coding rate ½ to the UE located in a cell boundary. In addition, compared to the existing high-speed power control method, the AMCS decreases an interference signal, thereby improving the average system performance.
The HARQ is a link control technique for correcting an error by retransmitting the errored data upon an occurrence of a packet error at an initial transmission. Generally, the HARQ is classified into Chase Combining (CC), Full Incremental Redundancy (FIR), and Partial Incremental Redundancy (PIR).
CC is a technique for transmitting a packet such that the whole packet transmitted at a retransmission is equal to the packet transmitted at the initial transmission. In this technique, a receiver combines the retransmitted packet with the initially transmitted packet that is previously stored in a buffer thereof by a predetermined method. By doing so, it is possible to increase reliability of coded bits input to a decoder, thus resulting in an increase in the overall system performance. Combining the two same packets is similar to repeated coding in terms of the effects, so it is possible to increase a performance gain by about 3 dB on average.
FIR is a technique for transmitting a packet comprised of only redundant bits generated from the channel encoder instead of the same packet, thus improving performance of a decoder in the receiver. That is, the FIR uses the new redundant bits as well as the initially transmitted information during decoding, resulting in a decrease in the coding rate, which thereby improves performance of the decoder. It is well known in coding theory that a performance gain by a low coding rate is higher than a performance gain by repeated coding. Therefore, the FIR is superior to the CC in terms of only the performance gain.
Unlike the FIR, the PIR is a technique for transmitting a combined data packet of the information bits and the new redundant bits at retransmission. Therefore, the PIR can obtain the similar effect as the CC by combining the retransmitted information bits with the initially transmitted information bits during decoding, and also obtain the similar effect as the FIR by performing the decoding using the redundant bits. The PIR has a coding rate slightly higher than that of the FIR, showing intermediate performance between the FIR and the CC. However, the HARQ should be considered in the light of not only the performance but also the system complexity, such as a buffer size and signaling of the receiver. As a result, it is not easy to determine only one of them.
The AMCS and the HARQ are separate techniques for increasing adaptability to the variations in the link environment. Preferably, it is possible to remarkably improve the system performance by combining the two techniques. That is, the transmitter determines a modulation technique and a coding rate proper for a downlink channel condition by the AMCS, and then transmits packet data according to the determined modulation technique and coding rate. Thus, upon failure to decode the data packet transmitted by the transmitter, the receiver sends a retransmission request. Upon receipt of the retransmission request from the receiver, the Node B retransmits the data packet by the HARQ.
FIG. 1 illustrates an existing transmitter for high-speed packet data transmission, wherein it is possible to realize various AMCS techniques and HARQ techniques by controlling a channel encoder 112.
Referring to FIG. 1, the channel encoder 112 is comprised of an encoder and a puncturer (not shown). When input data at a determined data rate is applied to an input terminal of the channel encoder 112, the encoder performs encoding in order to decrease a transmission error rate. Further, the puncturer punctures an output of the encoder according to a coding rate and an HARQ type previously determined by a controller 120, and provides its output to a channel interleaver 114. Since the future mobile communication system needs a powerful channel coding technique in order to reliably transmit high-speed multimedia data, the channel encoder 112 of FIG. 1 is realized by a turbo encoder with a mother coding rate R=⅙ and a puncturer 216, as illustrated in FIG. 2. It is known in the art that channel coding by the turbo encoder shows performance closest to the Shannon limit in terms of a bit error rate (BER) even at a low SNR. The channel coding by the turbo encoder is also adopted for the HSDPA and 1×EV-DV standardization by the 3GPP and the 3GPP2. The output of the turbo encoder can be divided into systematic bits and parity bits. The “systematic bits” refer to actual information bits to be transmitted, while the “parity bits” refer to a signal used to help a receiver correct a possible transmission error. The puncturer 216 selectively punctures the systematic bits or the parity bits output from the encoder, satisfying a determined coding rate.
Referring to FIG. 2, upon receiving one input data, the turbo encoder outputs the intact input data as a systematic bit stream X. The input data is also provided to a first channel encoder 210, and the first channel encoder 210 performs coding on the input data and outputs two different parity bit streams Y1 and Y2. In addition, the input data is also provided to an interleaver 212, and the interleaver 212 interleaves the input data. The intact interleaved input data is transmitted as an interleaved systematic bit stream X′. The interleaved input data is provided to a second channel encoder 214, and the second channel encoder 214 performs coding on the interleaved input data and outputs two different parity bit streams Z1 and Z2. The systematic bit streams X and X′ and the parity bit streams Y1, Y2, Z1 and Z2 are provided to the puncturer 216 in a transmission unit of 1, 2, . . . , N. The puncturer 216 determines a puncturing pattern according to a control signal provided from the controller 120 of FIG. 1, and performs puncturing on the systematic bit stream X, the interleaved systematic bit stream X′, and the four different parity bit streams Y1, Y2, Z1 and Z2 using the determined puncturing pattern, thus outputting desired systematic bits and parity bits.
As described above, the puncturing pattern used to puncture the coded bits by the puncturer 216 depends upon the coding rate and the HARQ type. That is, using the CC, it is possible to transmit the same packet at each transmission by puncturing the coded bits such that the puncturer 216 has a fixed combination of the systematic bits and the parity bits according to a given coding rate. Using the IR (either FIR or PIR), the puncturer 216 punctures the coded bits in a combination of the systematic bits and the parity bits according to the given coding rate at initial transmission, and punctures the coded symbols in a combination of various parity bits at each retransmission, thus decreasing in the overall coding rate. For example, using the CC with the coding rate ½, the puncturer 216 can continuously output the same bits X and Y1 for one input bit at initial transmission and retransmission, by fixedly using [1 1 0 0 0 0] in the order of the coded bits [X Y1 Y2 X′ Z1 Z2] as the puncturing pattern. Using the FIR, the puncturer 216 outputs the coded bits in the order of [X1 Y11 X2 Z21] at initial transmission and in the order of [Y21 Z21 Y12 Z12] at retransmission for two input bits, by using [1 1 0 0 0 0; 1 0 0 0 0 1] and [0 0 1 0 0 1; 0 1 0 0 1 0] as the puncturing patterns at initial transmission and retransmission, respectively. Meanwhile, though not separately illustrated, an R=⅓ turbo encoder adopted by the 3GPP2 can be realized by the first channel encoder 210 and the puncturer 216 of FIG. 2.
A packet data transmission operation by the AMCS system and the HARQ system realized by FIG. 1 will be described herein below. Before transmission of a new packet, the controller 120 of the transmitter determines a proper modulation technique and data rate based on the downlink channel condition information provided from the receiver. The controller 120 provides information on the determined modulation technique and coding rate to the channel encoder 112, a modulator 116 and a frequency spreader 118. A data rate in a physical layer depends upon the determined modulation technique and coding rate. The channel encoder 112 performs bit puncturing according to a given puncturing pattern after performing the encoding based on a signal from the controller 120, thereby finally outputting coded bits. The coded bits output from the channel encoder 112 are provided to the channel interleaver 114, in which they are subject to interleaving. Interleaving is a technique for preventing a burst error by randomizing the input bits to disperse data symbols into several places instead of concentrating the data symbols in the same place in a fading environment. For ease of explanation, the size of the channel interleaver 114 is assumed to be larger than or equal to the total number of the coded bits. The modulator 116 symbol-maps the interleaved coded bits according to the modulation technique previously determined by the controller 120 and a given symbol mapping technique. If the modulation technique is represented by M, the number of coded bits constituting one symbol becomes log2M. The frequency spreader 118 assigns multiple Walsh codes for the modulated symbols provided from the modulator 116, for high-speed data transmission corresponding to the data rate determined by the controller 120, and spreads the modulated symbols with the assigned Walsh codes. When a fixed chip rate and a fixed spreading factor (SF) are used in the high-speed packet transmission system, a rate of symbols transmitted with one Walsh code is constant. Therefore, in order to use the determined data rate, it is necessary to use multiple Walsh codes.
For example, when a system using a chip rate of 3.84 Mcps and an SF of 16 chips/symbol uses 16QAM and a channel coding rate ¾, a data rate that can be provided with one Walsh code becomes 1.08 Mbps. Therefore, when 10 Walsh codes are used, it is possible to transmit data at a data rate of a maximum of 10.8 Mbps.
It is assumed in the transmitter of the high-speed packet transmission system of FIG. 1 that the modulation technique and coding rate determined by the controller 120 at initial transmission of a data packet according to a channel condition is used even at retransmission. However, as described above, the high-speed data transmission channel is subject to a change in its channel condition even in a retransmission period by the HARQ due to the change in the number of UEs in a cell and the Doppler shift. Therefore, maintaining the modulation technique and the coding rate used at the initial transmission contributes to a reduction in the system performance.
For this reason, the ongoing HSDPA and 1×EV-DV standardizations consider an improved method for changing the modulation technique and the coding rate even in the retransmission period. For example, in a system using the CC as the HARQ, when the HARQ type is changed, a transmitter retransmits a part or the whole of the initially transmitted data packet, and a receiver partially combines the partially retransmitted packet with the whole of the initially transmitted packet, resulting in a reduction in the entire bit error rate of a decoder. Structures of the transmitter and the receiver are illustrated in FIGS. 3 and 4, respectively.
As illustrated in FIG. 3, the transmitter for the improved method further includes a partial Chase encoder 316 in addition to the transmitter of FIG. 1. Referring to FIG. 3, coded bits generated by encoding input data according to the given modulation technique and coding rate by a channel encoder 312 are provided to the partial chase encoder 316 after being interleaved by an interleaver 314. The partial Chase encoder 316 controls an amount of data (or the number of data bits) to be transmitted at retransmission among the interleaved coded bits based on information on a modulation technique used at initial transmission, a current modulation technique and the number of Walsh codes to be used, provided from the controller 322. A modulator 318 performs symbol-mapping on the coded bits output from the partial Chase encoder 316 according to a given modulation technique, and provides its output to a spreader 320. The spreader 320 assigns a needed number of Walsh codes among the Walsh codes available for the modulated symbols provided from the modulator 318, and frequency-spreads the modulated symbols with the assigned Walsh codes. Here, the channel coding rate at the retransmission is identical to the channel coding rate at the initial transmission, and the number of the Walsh codes to be used at the retransmission may be different from the number of the Walsh codes used at the initial transmission.
FIG. 4 illustrates a structure of a receiver corresponding to the transmitter illustrated in FIG. 3. The receiver further includes a partial Chase combiner 416 corresponding to the partial Chase encoder 316 of FIG. 3, in addition to the existing receiver. A despreader 412 despreads the modulated symbols transmitted from the transmitter with the same Walsh codes as used by the transmitter, and provides its output to a demodulator 414. The demodulator 414 demodulates the modulated symbols from the despreader 412 by a demodulation technique corresponding to the modulation technique used by the transmitter, and outputs a corresponding LLR (Log Likelihood Ratio) value to the partial Chase combiner 416. The LLR value is a value determined by performing soft decision on the demodulated coded bits. The partial Chase combiner 416 substitutes for the soft combiner in the existing receiver. This is because when the modulation used at the initial transmission is different from the modulation used at the retransmission, the packet combining is partially performed since an amount of the retransmitted data is different from an amount of the initially transmitted data. If the high-order modulation is used at retransmission, the partial Chase combiner 416 performs full combining on the entire packet. However, if the low-order modulation is used at retransmission, the partial Chase combiner 416 performs partial combining. The partial Chase combiner 416 provides the partially or fully combined coded bits to a deinterleaver 418. The deinterleaver 418 deinterleaves the coded bits from the partial Chase combiner 416 and provides the deinterleaved data to a channel decoder 420. The channel decoder 420 decodes the deinterleaved coded bits according to a given decoding technique. Though not illustrated in FIG. 4, the receiver performs CRC (Cyclic Redundancy Check) checking on the decoded information bits, and transmits an ACK (Acknowledge) or a NACK (Negative Acknowledge) signal to a Node B according to the CRC checking results, thereby requesting transmission of new data or retransmission of the errored packet.
FIG. 5A illustrates a change in a size of the packet encoded by the partial Chase encoder 316 illustrated in FIG. 3 according to a change in the modulation technique at initial transmission and retransmission and a change in number of available codes. It is assumed herein that a turbo code rate is ½ and the number of available codes used at retransmission is reduced to 3, which is smaller than half of the 8 available codes used at initial transmission. If a modulation order used at retransmission is higher than a modulation order used at initial transmission, only a part of the initially transmitted packet is retransmitted. For example, as illustrated in (a-2) of FIG. 5A, if a modulation technique is changed from Mi=QPSK at initial transmission to Mr=16QAM at retransmission, the number of coded bits needed per code during retransmission becomes twice the number of coded bits needed per code during initial transmission. However, since the number of codes assigned during retransmission is smaller than half of the number of codes assigned at initial transmission, only a part of the initially transmitted packet is retransmitted. In this case, among the data blocks transmitted through a total of 8 codes during initial transmission, only the data blocks A, B, C, D, E, and F corresponding to the first 6 codes are transmitted through 3 available codes during retransmission. In addition, as illustrated in (a-1) of FIG. 5A, if a modulation technique used at retransmission is identical to a modulation technique used at initial transmission (Mi=Mr), a size of data that can be transmitted is reduced in proportion to the reduced number of codes. Therefore, among the data blocks transmitted through the 8 codes during initial transmission, only the data blocks A, B, and C corresponding to the first 3 codes are transmitted through 3 available codes during retransmission.
FIG. 5B illustrates how the partial Chase combiner 416 combines a data packet transmitted through the partial Chase encoder 316 during initial transmission and retransmission. For example, as illustrated in (b-2) of FIG. 5B, if a modulation technique is changed from Mi=QPSK to Mr=16QAM, data blocks that can be retransmitted due to a change in number of codes are A, B, C, D, E and F among the initially transmitted data blocks. Therefore, the data blocks A, B, C, D, E, and F are partially soft-combined with the initially transmitted data blocks A to H, thereby increasing the reliability of a received signal. In addition, as illustrated in (b-1) of FIG. 5B, if a modulation technique used at retransmission is identical to a modulation technique used at initial transmission (Mi=Mr), a retransmitted data packet corresponds to the initially transmitted data blocks A to C. Therefore, the partial Chase combiner 416 performs partial Chase combining on the initially transmitted packet and the retransmitted packet. Here, it should be noted that although a size of the combined data block is smaller as compared with the case of (b-2), since the low-order modulation is used, reliability of combined retransmission data is relatively high. Therefore, performance is not always linearly determined according to the size of the combined partial packet.
In FIGS. 5A and 5B, a case where the number of codes is increased during retransmission is not taken into consideration because when the modulation order used at retransmission is higher than or equal to the modulation order used at initial transmission, if the number of codes assigned for retransmission is larger than the number of codes assigned for initial transmission, the entire packet can be combined. In this case, it is preferable to use the same modulation technique instead of changing the modulation technique to a high-order modulation technique.
FIGS. 6A and 6B illustrate operations of the partial Chase encoder 316 and the partial Chase combiner 416, respectively, when the number of codes used at retransmission is increased to 6, compared with the 4 codes used at initial transmission.
Referring to (a-2) of FIG. 6A, if a modulation technique is changed from Mi=16QAM at initial transmission to Mr=QPSK at retransmission, data blocks transmitted through 2 codes during retransmission correspond to the data blocks transmitted through one code during initial transmission. Therefore, among the initially data blocks, data blocks A, B, and C corresponding to first 3 codes are transmitted through the assigned 6 codes during retransmission. The data blocks A, B, and C are finally partially soft-combined with the initially transmitted data blocks at the receiver, as illustrated (b-2) of FIG. 6A.
Referring to (a-1) of FIG. 6A, if a modulation technique at retransmission is identical to a modulation technique at initial transmission (Mi=Mr), data blocks A, B, C, D, A, and B, which amount to 1.5 times the initially transmitted data blocks, can be transmitted during retransmission. Therefore, as illustrated in (b-1) of FIG. 6B, by one transmission, the receiver can obtain two-soft combining effect for the data blocks A and B, and one-soft combining effect for the data blocks C and D. That is, an effect of simultaneously performing full combining several times can be obtained, thus increasing the system performance. However, as described above, the size of the combined partial packet is not always proportional to the performance. This is because a process of combining the entire packet using the same modulation technique in a bad channel condition and a process of combining the partial packet using the low-order modulation technique have advantages and disadvantages. In FIGS. 6A and 6B, a case where a modulation order used at retransmission is higher than a modulation order used at initial transmission is not taken into consideration because the number of codes is increased due to the worsened channel condition during retransmission, the transmitter allowed to use the same modulation technique as used at initial transmission, as described in conjunction with (a-1) of FIG. 6A.
In a high-speed packet transmission system in which the number of codes available for retransmission is variable and the CC is used for the HARQ, if the partial Chase encoder 316 and the partial Chase combiner 416 illustrated in FIGS. 3 and 4 are used, it is possible to increase the system performance by more actively coping with a change in the channel environment by changing the modulation technique even at retransmission. However, as illustrated in (b-2) of FIG. 5B and (b-2) of FIG. 6B, the partial combining on the entire transmission packet contributes to a decrease in the bit error rate, but fails to satisfactorily contribute to a reduction in the frame error rate. This is because the output of the channel interleaver 314 of FIG. 3 is a random combination of the systematic bits and the parity bits from the channel encoder 312. That is, if the packet size at retransmission is smaller than the packet size at initial transmission, the combining cannot be performed on all of the information bits, so the combining effect occurs randomly in a bit unit. In particular, there is a demand for a new method for remarkably reducing a frame error rate by compensating all of the information bits using the feature that the turbo code should be transmitted in combination of the systematic bits and the parity bits even when the system using the CC is required to transmit a smaller packet at retransmission than at initial transmission.